Linking trait variation to growing conditions: the seed ecology of the invasive Aegilops triuncialis

Background/Question/Methods

Adaptive plastic responses in populations of invasive plant species allow for establishment and persistence despite the lack of genetic matching to the particular environment.  The capacity of a plant to successfully invade a habitat to which it is not adapted is very likely predicated on seed traits, especially for annual plants, and those seed traits are predicted to exhibit plastic responses to the conditions in the invaded habitat.  To investigate this prediction, we compared annual variation in seed traits and germination patterns to the growing conditions at the time of seed development of Aegilops triuncialis (barbed goatgrass), an invasive and spreading annual grass in northern California.  The data were collected from 72 sample populations from 24 locations across 12 years.  Using spikelets with dimorphic seed pairs, we measured seed mass, germination fractions, seed viability, and strength of induced dormancy to investigate plastic variation within and among populations, across two soil types, and among the two putative maternal haplotypes.  We predicted that variation in seed traits would be correlated with environmental variables that influence the length of the growing season.

 Results/Conclusions

We found that seed mass variation and small to large seed mass ratios are tightly constrained within the species.  However, masses of both seed types showed a 3-fold range among populations, varied by ~20% within populations among years, and seed mass was positively correlated with longer growing seasons.  The strength of induced seed dormancy in the small seed type was correlated with the mass of the large seed and to late season precipitation which suggests that longer growing seasons influence the strength of maternal and sibling chemical signals that induce dormancy in the small seeds of A. triuncialis.  Thus, variation in small seed germination fractions varies with the growing conditions experienced by the maternal plant. In combination with the greater longevity of the small seed type, these results suggest that plasticity in seed traits influencing germination in this species may contribute to seed bank formation, which is atypical of invasive annual grasses in California, and this may contribute to the rapid spread of A. triuncialis.